The invention relates to a process for the production of glazing with a high transmittance in the visible spectral range, and with a high reflectance for thermal radiation. More particularly, the invention relates to producing a curved glazing consisting of a glass substrate having first and fifth layers of oxide, second and fourth layers of protective metal, and a third layer of silver therebetween.
Such glazing is to transmit as much as possible of visible light and block as much as possible of the invisible part of the spectrum of solar radiation. In summer this is to prevent the undesired thermal radiation of sunlight into rooms. Especially in motor vehicles such thermal radiation is particularly undesirable on account of the sloping and therefore very large windshields and rear windows.
DE-OS No. 33 07 661 to which U S. Pat. No. 4,548,691 to Dietrich corresponds, and EP-OS No. 104 870 have disclosed the concept of sandwiching between two oxide coatings a silver coating and a very thin metal coating applied to the silver, the thin metal coating protecting the silver coating against chemical-thermal attack during the production of the topmost oxide coating. If, however, such a sandwich system is exposed to temperatures above 150.degree. C., a diffusion of the silver into the contiguous oxide and/or metal coatings occurs, and in this case a great increase of the surface resistance and a corresponding lowering of the transmittance of the system is to be observed, i.e., two of the important properties of the layered system are degraded.
EP-PS 35 906 discloses the concept of arranging a thin metallic coating on one side or both sides of a silver coating and sandwiching this coating system likewise between two oxide layers. The thin metallic layer is intended to achieve greater long-term stability, but the studies are limited to a temperature range up to 120.degree. C. Since the absorptance of the metallic layers causes an impairment of the optical transparency of the coated substrates, the total thickness of the metallic layers for the achievement of a particular transmittance is limited, and therefore it is to be preferred to apply only one metallic coating to the side of the silver coating facing away from the substrate so as to limit the degradation of the transmittance. It was found however that, when substrates of mineral glass and such a layered system are heated to temperatures markedly higher than 150.degree. C., a degradation of the properties of the layers occurred, resulting not only in an increase in the transmittance in the visible range, desirable in itself, but also by an undesired increase of the surface resistance, so that the on-board voltage of a normal motor vehicle no longer suffices to perform sufficient heating in winter. Furthermore, spotting and undesired light scatter were observed, which evidently are to be attributed to a partial agglomeration as well as to a partial diffusion of the silver into the oxide layers.
This behavior leads only to the conclusion that the thickness of the metallic layer or layers is no longer sufficient for the effective protection of the silver layer, especially at higher temperatures, in view of the targeted high transmittance in the visible range which is to be attributed in part to requirements of law (motor vehicle windshields).
It is known to produce curved or protuberant glazing with an approximately similar spectral behavior by applying a silver coating sandwiched between other coatings to prepared glass substrates. This, however, calls for special coating apparatus allowing for the different shapes of the substrates, and the compensation of the effects of varying distances between units of area of the substrate and the coating sources and of the varying angles at which the coating material impinges by means of complicated masking.
The invention is therefore addressed to the problem of devising a process of the kind specified in the beginning, whereby curved or protuberant glazing can be produced in a more economical manner.